1. Field of the Invention
The invention relates to a process for the regeneration of used sulfuric acid which is contaminated by the secondary components methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organic silicon compounds.
2. Description of the Related Art
Chloromethane is used in the direct synthesis of methylchlorosilanes. The necessary chloromethane is usually produced from methanol and hydrogen chloride, wherein, as secondary products, water and, in small amounts, dimethyl ether are formed. The resultant chloromethane, for production of methylchlorosilanes, must be carefully dried and freed from dimethyl ether.
The hydrogen chloride used for the production of chloromethane mostly originates from hydrolysis reactions of methylchlorosilanes. Therefore, in the chloromethane produced, secondary components of methylchlorosilanes, such as aliphatic and olefinic hydrocarbons and chlorinated hydrocarbons, may also be found. If hydrogen chloride from hydrolysis reactions of methylchlorosilanes is used directly in chloromethane production without intermediate purification, additional organic silicon compounds can pass into the product, in accordance with the vapor pressure.
The chloromethane can be freed from the abovementioned impurities by scrubbing with concentrated sulfuric acid. In this case water, dimethyl ether, olefins and organic silicon compounds are completely removed, and chlorinated hydrocarbons and hydrocarbons are removed in part. The used sulfuric acid, at a concentration of about 75% by weight or less, must be discharged, since its absorption capacity for dimethyl ether is exhausted.
The application DE 25 03 610 A1 describes a process for purifying sulfuric acid contaminated by methylsulfuric acid. In this process, used sulfuric acid originating from the purification of chloromethane is diluted with 10 to 25% by weight of water, based on the sulfuric acid content. Subsequently, steam is introduced, wherein a temperature of 170 to 180° C. is achieved, at which methylsulfuric acid is hydrolyzed to methanol and sulfuric acid, and methanol is distilled off.
The process according to DE 25 03 610 A1 is not applicable to sulfuric acid which, in addition to methylsulfuric acid, dimethyl ether and methanol, still contains additional impurities as described previously, since, owing to the high temperature, cracking processes occur. The resultant carbon is solid like coke and leads to rapid blockage of equipment parts.
In the booklet “Schott Engineering, 1987” from Schott Engineering GmbH, Mainz, pages 12 to 17, a process is described for concentration and purification of waste sulfuric acid, in which oxidizing agents are added in a problem-specific manner to the waste acid.
Oxidation of organic impurities is complex at high loads, since large amounts of oxidizing agents must be added. The oxidation of organosilicon compounds leads to colloidal silicic acid. The filtration, owing to the aggressive medium and owing to the small particle size, is a relatively large problem. Furthermore, contacting relatively large amounts of an oxidizing agent with the dimethyl ether-containing sulfuric acid is a considerable safety risk.
The subsequent addition of oxidizing agents to the process described in DE 25 03 610 A1 is also not expedient, since the coarse coke particles, owing to the small surface area in comparison with the amount thereof, are oxidized only with difficulty, and slowly.
European patent EP 0 784 031 B1 describes a process which removes the above impurities, wherein the sulfuric acid for the purification is diluted in a first step with water and is stripped with steam at a concentration of 45 to a maximum of 55% by weight, is treated in a second purification step by adding an oxidizing agent at 80-130° C. and is subsequently concentrated to at least 90% by weight, in order to reuse it for the chloromethane purification. However, this method has some disadvantages. The used sulfuric acid coming from the MeCl purification already contains solid particles of carbon black and coke. These solids are not removed by the stripping. The oxidation of solids in the roughly 50% strength sulfuric acid proceeds only incompletely. The solids concentrate with time, so that a controlled feed of fresh sulfuric acid is constantly necessary. In addition, there is a relatively high consumption of oxidizing agents, since at the high temperatures, increased autodecomposition of the oxidizing agent occurs and the oxidizing agent is therefore not available for the oxidation.